Renewable energy power generation systems

ABSTRACT

We describe a modular adjustable power factor renewable energy inverter system. The system comprises a plurality of inverter modules having a switched capacitor across its ac power output, a power measurement system coupled to a communication interface, and a power factor controller to control switching of the capacitor. A system controller receives power data from each inverter module, sums the net level of ac power from each inverter, determines a number of said capacitors to switch based on the sum, and sends control data to an appropriate number of the inverter modules to switch the determined number of capacitors into/out of said parallel connection across their respective ac power outputs.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/446,319, filed Jul. 29, 2014, now issued as U.S.Pat. No. 9,209,710. U.S. patent application Ser. No. 14/446,319 is acontinuation application of U.S. patent application Ser. No. 13/769,275,filed Feb. 15, 2013, now issued as U.S. Pat. No. 8,823,212. U.S. patentapplication Ser. No. 13/769,275 is a continuation application of U.S.patent application Ser. No. 13/310,691, filed Dec. 2, 2011, now issuedas U.S. Pat. No. 8,391,032. U.S. patent application Ser. No. 13/310,691claims benefit of an earlier-filed United Kingdom Patent Application1120367.6, filed Nov. 25, 2011. U.S. patent application Ser. No.14/446,319, now U.S. Pat. No. 9,209,710, U.S. Pat. No. 8,823,212, U.S.Pat. No. 8,391,032, and United Kingdom Patent Application 1120367.6 areincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to systems and methods for generation ofelectrical power from renewable sources, in particular solarphotovoltaic (PV) power generation systems. More particularly theinvention relates to VAR (volt-ampere reactive) control in such systems.

BACKGROUND TO THE INVENTION

As the skilled person will be aware, VAR power refers to an imaginarycomponent of the total power supplied by a power generation system,resulting from driving a reactive load such that the current and voltageare out of phase with one another. Driving an inductive load causes thecurrent to lag the voltage, and vice versa for a capacitive load. Thusthe reactive component of power can be compensated by addingcapacitance/inductance as needed. The degree of reactive power isspecified by the power factor cos φ where φ is the phase angle betweenthe current and the voltage.

Examples of reactive power control systems can be found in, inter alia:US2010/246226; U.S. Pat. No. 7,142,997; US2004/164718; US2002/180408;U.S. Pat. No. 4,683,529; U.S. Pat. No. 4,545,997; U.S. Pat. No.4,554,502; and U.S. Pat. No. 4,453,207.

New grid requirements are being imposed in regions where powergeneration from renewable sources, in particular solar, is becoming alarger proportion of total power generation. In particular these placerequirements on VAR control for reactive power correction wherediffering sources supplying power to the grid could otherwise force thegrid into an under-excited or over-excited mode during peak and off-peakcycles of the overall system. Thus, for example, the ability to absorbor export VLRs is required by new standards in Germany, in particularVDE4105, which will be mandatory for new products from January 2012.Thus the ability to react and correct and/or actively damp VARs ishighly desirable.

SUMMARY OF THE INVENTION

According to the invention there is therefore provided a modularadjustable power factor renewable energy inverter system, the systemcomprising: a plurality of inverter modules each having a dc power inputand an ac power output; a common grid power feed coupled to each saidinverter module ac power output; and a system controller having acommunications link to each said solar inverter module to receive powerdata from the inverter module and to send control data to the inverter;wherein each said solar inverter module further comprises: a seriescombination of a capacitor and a controllable switching device connectedin parallel across said ac power output; a communication interface forcommunicating with said system controller; a power measurement systemcoupled to said communication interface to measure one or moreparameters relating to a net level of ac power provided by said invertermodule to said common grid power feed, and to provide power data,defining said net level of ac power provided, to said communicationsinterface for transmission to said system controller; and a power factorcontroller, coupled to said communications interface to receive controldata from said system controller, and coupled to said controllableswitching device to control said switching device in response to saidcontrol data to switch said capacitor into/out of parallel connectionacross said ac power output of said inverter module; and wherein saidsystem controller is configured to: receive said power data from eachsaid inverter module; sum the net level of ac power from each saidinverter module to determine a total net level of ac power provided bysaid plurality of inverter modules to said common grid power feed;determine a number of said capacitors to switch into/out of parallelconnection across their respective said ac power outputs responsive tosaid total net level of ac power; and send control data to said invertermodules to switch said determined number of capacitors into/out of saidparallel connections.

Embodiments of the above described system facilitate the implementationof a flexible and cost effective renewable power system, in particular asolar inverter system. There is no need for bi-directional current flowthrough the output, unfolding stage of the inverter. Also, because theswitched capacitors are distributed amongst the solar inverter modulesrather than being centralised, the system can easily be upgraded orotherwise modified to change the power generation capability, simply byadding or removing inverter modules. This is because each module isprovided with separate VAR control whilst the overall VAR of theensemble of modules is managed by a centralised system controller. Inthis manner very tight requirements on the overall system power factorcan be met in a cost-effective manner. In some preferred embodiments theensemble of modules and system controller are connected by a wirelesslocal area network, for example a Zigbee™ network, to carry the powerand control data by, thereby facilitate installation and upgrade of thesystem as required.

In some preferred implementations of the system the system controllerdetermines a fraction of a total maximum (rated) ac power output of thesystem being provided by the set of inverter modules at any one time.The controller then uses this information to determine the number ofcapacitors to switch in parallel with the ac outputs of the respectivemodules, then sending control signals to that number of inverter modulesto switch the capacitors into parallel connection across theirrespective ac power outputs. The switching in/out of a capacitor mayemploy on/off control to switch the capacitor in or out, or may employphase control, to control a proportion of an ac cycle for which thecapacitor is switched in, to provide a variable degree of power factorcompensation.

In preferred implementations the system controller applies a thresholdto the determined fraction of maximum power being provided such thatuntil this is reached no capacitors are switched in. In embodiments thisthreshold fraction is 0.5 (50%). Beyond this point additional invertermodules are controlled to incrementally increase the number of invertermodules applying a capacitor across their respective ac output, thusincreasing the total applied capacitance in a stepwise fashion tocontrol the displacement power factor of the system. In this way inprinciple the desired power factor response can be approximated to anarbitrary degree of accuracy by employing sufficiently small steps. Inpractice the number of steps (inverters/capacitors) depends upon thetolerance permitted for the system.

In embodiments the change in displacement power factor above thethreshold is approximately linear. This is because, in embodiments, aninverter produces a sinusoidal voltage/current waveform in phase withthe grid so that the power factor of the inverter by itself—that is, notincluding the capacitor—is close to unity. In practice, however, thepower factor is not precisely unity and in embodiments the systemcontroller may include a lookup table storing data defining the numberof inverter modules/capacitors to switch in/out in response to thefraction of the total power being supplied. (The skilled person willappreciate that the system is quasi-linear in displacement power factorrather than phase angle since power factor is dependent on cos φ).

In embodiments the system employs open loop control of the displacementpower factor—that is the system does not need to measure a power factorin order to compensate to provide the desired response: instead thesystem controller merely senses (measures) the power output level ofeach inverter and then controls the overall displacement power factorbased on a percentage of the total power available. Thus each invertermodule only needs to know its own output power, and to report this tothe system controller (gateway), which adds up the individual powercontributions to determine the overall number of capacitors to apply fordissipation power factor compensation. Broadly speaking, more capacitorsare employed if the inverters are providing a greater percentage of thetotal potential power output, subject to a 50% break point—that is nocapacitors are applied below this power level.

In embodiments an individual inverter knows its net power output becausethe RMS voltage is known (or measured) and because the output currentamplitude is known because this is controlled. Thus an inverter alsoknows the percentage of the full power output current it is providingand therefore the power measurement parameter or parameters providedback to the system controller may either comprise an absolute value ofan output current or a percentage of a total potential output current,or an absolute output power or a percentage of a total output power.Based upon this the system controller, via the commutations network, canthen control the local power factor controllers switching the capacitorsin/out thus although a control loop is employed in the sense that aninverter measures power output or a parameter dependent upon this,provides this to the system controller, and then receives back controldata for switching its capacitor in/out, the system is open loop in thesense that the displacement power factor per se is not measured. Thisapproach significantly simplifies the overall system design and further,because the capacitors are distributed amongst the ensemble of modules,there are further savings in cost and physical size, as well asimprovements in reliability and efficiency.

The above described techniques can be employed with either a singlephase or a three phase solar power generation system; in a three phasesystem preferably one set of solar inverter modules is provided for eachphase. The system controller may then implement a separate control loopfor each phase, in effect controlling the dissipation power factor foreach set of inverter modules independently.

In principle the device switching a capacitor in/out of parallelconnection across the ac output of an inverter module may be anelectromechanical device. However in some preferred implementations atriac is employed as the electronic switch as this is particularlyeffective in handling power surges. Where a triac is employed this maybe turned on by a single pulse (or current) timed to be provided whenthe current through the device is increasing (so that it latches), or bya train of pulses (which relaxes the timing requirements). Turning thedevice on with a single pulse can, in embodiments, be preferable as lesspower is required. The skilled person will appreciate that suitableelectronic switching devices are not limited to triacs—for example apair of back to back MOSFETs may alternatively be employed.

Preferably where an electronic switching device is employed, this isturned on (or off) substantially at the peak of the grid voltage. Thisis because the current in the capacitor is 90° out of phase with thisvoltage and thus the peak grid voltage coincides with a capacitorcurrent of substantially zero. However initially the electronicswitching device may be switched in at a zero-crossing of the gridvoltage, to reduce stress on the VAR control capacitor(s).

Thus where a triac is employed, preferably the triac is first switchedon at a zero-crossing of the grid voltage. In embodiments of thisapproach the power factor controller may configured to generate a trainof pulses for at least one complete cycle of the grid power to controlthe triac on, preferably generating the pulse train for a plurality ofcycles of the grid power, for example more than 5 or 10 complete cycles.Then, after allowing time for the switching transient to die down, thetriac may be driven by a single pulse or pulse train at substantially apeak voltage of the ac power.

In a related aspect the invention provides a method of controlling powerfactor in a renewable energy inverter system, the method comprising:providing a plurality of inverter modules each having a dc power inputand an ac power output; coupling the ac power output of each saidinverter module to a common grid power feed; providing each saidinverter module with at least one reactive element switchable to beconnected to a said ac power output; and controlling a dissipation powerfactor of said inverter system by: monitoring the ac power provided tosaid common grid power feed by each said inverter module to determine atotal ac power provided to said common grid feed by said plurality ofinverter modules; determining a number of said inverter modules tocontrol, dependent on a proportion of a maximum power output of saidinverter system represented by said total ac power provided to saidcommon grid feed; and controlling said determined number of invertermodules to switch said reactive element into the respective said acpower output to control the displacement power factor of said invertersystem.

As previously described, embodiments of this method facilitate adistributed, extendable solar inverter system with accurate power factorcontrol. In some preferred implementations the inverter includes anoutput stage with a current source inverter topology, which can providea lagging power factor. Then the reactive element may be a capacitorarranged so that it can be switched into/out of parallel connection withthe ac power output of its inverter. However in principle similartechniques to those described above may also be employed with a seriesinductance.

In some preferred embodiments of the technique each inverter has a justa single switched capacitance (albeit this may be implemented asmultiple capacitors connected together), but in principle multipleswitched capacitances may be provided for each inverter module. Againthe switching in/out of a capacitor may employ on/off control to switchthe capacitor in or out, or may employ phase control, to control aproportion of an ac cycle for which the capacitor is switched in, toprovide a variable degree of power factor compensation.

The method of controlling the power factor of the system may include adesign step of selecting one or both of a number of solar invertermodules (and their respective ratings), and a number of switchablecapacitors (where a capacitor may be implemented by a plurality ofparallel connected physical devices) in each module to provide thefacility for stepwise control of the displacement power factor to withina desired tolerance limit, for example +/−1%. In embodiments thetolerance limit may be better than 5%, 3%, 2%, or 1% (that is, betterthan +/−2.5%, 1.5%, 1%, or 0.5%).

In operation, as previously described, in embodiments there is athreshold level of summed power from the inverter modules below whichnone of the capacitors of the inverter modules are connected acrosstheir respective ac outputs. This may be, for example, 50% of theavailable maximum power output.

As previously mentioned, although some preferred implementations of thetechnique are employed in a solar photovoltaic power conversion system,in principle similar techniques may also be employed in other renewablepower generation systems, in particular where multiple, relatively smallpower sources are connected together to provide power to the grid.

In a related aspect the invention provides a modular adjustable powerfactor renewable energy inverter system, the system comprising: aplurality of inverter modules each having a dc power input and an acpower output, each further comprising at least one reactive elementswitchable to be connected to a said ac power output; a common gridpower feed coupled to each said inverter module ac power output; and apower factor controller for controlling a dissipation power factor ofsaid inverter system, wherein said power factor controller is configuredto: monitor the ac power provided to said common grid power feed by eachsaid solar inverter module to determine a total ac power provided tosaid common grid feed by said plurality of inverter modules; determine anumber of said inverter modules to control, dependent on a proportion ofa maximum power output of said inverter system represented by said totalac power provided to said common grid feed; and control said determinednumber of inverter modules to switch said reactive element into therespective said ac power output to control the displacement power factorof said inverter system.

In embodiments of the system, the system controller may be implementedin hardware, or in software for example on a microcontroller or digitalsignal processor, or in a combination of the two. Thus the inventionalso provides processor control code on a physical data carrier such asa disk, configured to implement the system controller. his code (and/ordata) may comprise code for controlling a processor or code for settingup or controlling a ASIC, or FPGA, or code for a hardware descriptionlanguage.

As the skilled person will appreciate, the functions of the systemcontroller may either be implemented at one point in the system or maybe distributed between a plurality of coupled components incommunication with one another. For example the functions of the systemcontroller may be shared amongst the inverter modules.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further aspects ofthe invention will now further described, by way of example only, withreference to the accompanying figures in which:

FIGS. 1a and 1b show, respectively, power factor requirements accordingto German standard VDE4105, and an outline block diagram of an examplepower conditioning unit;

FIGS. 2a and 2b show details of a power conditioning unit of the typeshown in FIG. 1 b;

FIGS. 3a and 3b show details of a further example of solar photovoltaicinverter in which an input power converter incorporates an LLC resonantpower converter;

FIGS. 4a to 4c show, respectively, single phase and three phase modularadjustable power factor solar converter systems according to embodimentsof the invention, and an example illustration of a residential propertyfitted with the system;

FIGS. 5a-5c show, respectively, block diagrams showing increasing detailof a solar inverter module for use with the system of FIG. 4;

FIG. 6 shows a flow diagram of a procedure for implementation by asystem controller of the system of the system of FIG. 4; and

FIGS. 7a and 7b show, respectively, a graph of system displacement powerfactor against the target requirement of FIG. 1 a, and the power factorerror in tracking the curve of FIG. 1 a.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

VAR control is required where grid requirements mandate leading orlagging control over the output of any grid connected power source.Typical requirements provide limits in the range.

FIG. 1a shows VDE4105 requirements in which the power factor of systemsabove 3.68 kVA must be continuously controlled based on percentage offull load system power. The power factor for systems above 13.8 kVAfollows the 0.9 curve, those below 13.8 kVA follow the 0.95 curve.

Further, power sources must be capable of being programmed to a specificpower factor in overexcited and under-excited conditions with a latencyof 10 seconds. We address this by employing feedback from thesub-source, a micro-inverter, in the context of a combined system ofmultiple micro-inverters' connected to grid. In embodiments the abilityto close this feedback loop and to control power factor (per phase in athree phase system) to within 0.01 (cos φ) enables high accuracy andcontrol at the grid connection point.

Power Conditioning Units

By way of background and context, to assist in understanding theoperation of embodiments of the invention we first describe an examplephotovoltaic power conditioning unit. Thus in FIG. 1b shows photovoltaicpower conditioning unit of the type we described in WO2007/080429. Thepower converter 1 is made of three major elements: a power converterstage A, 3, a reservoir (dc link) capacitor C_(dc) 4, and a powerconverter stage B, 5. The apparatus has an input connected to a directcurrent (dc) power source 2, such as a solar or photovoltaic panel array(which may comprise one or more dc sources connected in series and/or inparallel). The apparatus also has an output to the grid main electricitysupply 6 so that the energy extracted from the dc source is transferredinto the supply. Capacitor C_(dc) is preferably non-electrolytic, forexample a film capacitor.

The power converter stage A may be, for example, a step-down converter,a step-up converter, or it may both amplify and attenuate the inputvoltage. In addition, it generally provides electrical isolation bymeans of a transformer or a coupled inductor. In general the electricalconditioning of the input voltage should be such that the voltage acrossthe dc link capacitor C_(dc) is always higher than the grid voltage. Ingeneral this block contains one or more transistors, inductors, andcapacitors. The transistor(s) may be driven by a pulse width modulation(PWM) generator. The PWM signal(s) have variable duty cycle, that is,the ON time is variable with respect to the period of the signal. Thisvariation of the duty cycle effectively controls the amount of powertransferred across the power converter stage A.

The power converter stage B injects current into the electricity supplyand the topology of this stage generally utilises some means to controlthe current flowing from the capacitor C_(dc) into the mains. Thecircuit topology may be either a voltage source inverter or a currentsource inverter.

FIG. 2 shows details of an example of a power conditioning unit of thetype shown in in FIG. 1 b; like elements are indicated by like referencenumerals. In FIG. 2a Q1-Q4, D1-D4 and the transformer form a dc-to-dcconversion stage, here a voltage amplifier. In alternative arrangementsonly two transistors may be used; and/or a centre-tapped transformerwith two back-to-back diodes may be used as the bridge circuit.

In the dc-to-ac converter stage, Q9, D5, D6 and Lout perform currentshaping. In alternative arrangements this function may be located in aconnection between the bridge circuit and the dc link capacitor: D₆ actsas a free-wheeling diode and D₅ prevents current form flowing back intothe dc-link. When transistor Q₉ is switched on, a current builds upthrough L_(out). When Q₉ is switched off, this current cannot return tozero immediately so D₆ provides an alternative path for current to flowfrom the negative supply rail (D₅ prevents a current flowing back intothe dc-link via the body diode in Q₉ when Q₉ is switched off). Currentinjection into the grid is controlled using Q₉: when Q₉ is turned on thecurrent flowing through L_(out) increases and decreases when it isturned off (as long as the dc-link voltage is maintained higher than thegrid voltage magnitude). Hence the current is forced to follow arectified sinusoid which is in turn unfolded by the full-bridge output(transistors Q₅ to Q₈). Information from an output current sensor isused to feedback the instantaneous current value to a control circuit:The inductor current, i_(out), is compared to a reference current,i_(ref), to determine whether or not to switch on transistor Q₉. If thereference current is higher than i_(out) then the transistor is turnedon; it is switched off otherwise. The reference current, i_(ref), may begenerated from a rectified sinusoidal template in synchronism with theac mains (grid) voltage.

Transistors Q5-Q8 constitutes an “unfolding” stage. Thus thesetransistors Q5-Q8 form a full-bridge that switches at line frequencyusing an analogue circuit synchronised with the grid voltage.Transistors Q5 and Q8 are on during the positive half cycle of the gridvoltage and Q6 and Q7 are on during the negative half cycle of the gridvoltage.

Thus in embodiments the power conditioning unit comprises a genericdc-ac-dc that provides voltage amplification of the source to above thegrid voltage, and isolation, and a current source inverter (CSI)connected to the mains. The current injection is regulated using currentshaping (current-control) in the inductor of the CSI via theintermediate buck-type stage. (This is described further in ourGB2415841B, incorporated by reference).

Control (block) A of FIG. 1b may be connected to the control connections(e.g. gates or bases) of transistors in power converter stage A tocontrol the transfer of power from the dc energy source. The input ofthis stage is connected to the dc energy source and the output of thisstage is connected to the dc link capacitor. This capacitor storesenergy from the dc energy source for delivery to the mains supply.Control (block) A may be configured to draw such that the unit drawssubstantially constant power from the dc energy source regardless of thedc link voltage V_(dc) on C_(dc).

Control (block) B may be connected to the control connections oftransistors in the power converter stage B to control the transfer ofpower to the mains supply. The input of this stage is connected to thedc link capacitor and the output of this stage is connected to the mainssupply. Control B may be configured to inject a substantially sinusoidalcurrent into the mains supply regardless of the dc link voltage V_(dc)on C_(dc).

The capacitor C_(dc) acts as an energy buffer from the input to theoutput. Energy is supplied into the capacitor via the power stage A atthe same time that energy is extracted from the capacitor via the powerstage B. The system provides a control method that balances the averageenergy transfer and allows a voltage fluctuation, resulting from theinjection of ac power into the mains, superimposed onto the average dcvoltage of the capacitor C_(dc). The frequency of the oscillation can beeither 100 Hz or 120 Hz depending on the line voltage frequency (50 Hzor 60 Hz respectively).

Two control blocks control the system: control block A controls thepower stage A, and control block B power stage B. An exampleimplementation of control blocks A and B is shown in FIG. 2 b. In thisexample these blocks operate independently but share a commonmicrocontroller for simplicity.

In broad terms, control block A senses the dc input voltage (and/orcurrent) and provides a PWM waveform to control the transistors of powerstage A to control the power transferred across this power stage.Control block B senses the output current (and voltage) and controls thetransistors of power stage B to control the power transferred to themains. Many different control strategies are possible. For exampledetails of one preferred strategy reference may be made to our earlierfiled WO2007/080429 (which senses the (ripple) voltage on the dclink)—but the embodiments of the invention we describe later do not relyon use of any particular control strategy.

In a photovoltaic power conditioning unit the microcontroller of FIG. 2bwill generally implement an algorithm for some form of maximum powerpoint tracking. In embodiments of the invention we describe later thisor a similar microcontroller may be further configured to controlwhether one or both of the dc-to-dc power converter stages areoperational, and to implement “soft” switching off of one of thesestages when required. The microcontroller and/or associated hardware mayalso be configured to interleave the power transistor switching,preferable to reduce ripple as previously mentioned.

Now referring to FIG. 3 a, this shows a further example of a powerconditioning unit 600. In the architecture of FIG. 3 a photovoltaicmodule 602 provides a dc power source for dc-to-dc power conversionstage 604, in this example each comprising an LLC resonant converter.Thus power conversion stage 604 comprises a dc-to-ac (switching)converter stage 606 to convert dc from module 602 to ac for atransformer 608. The secondary side of transformer 608 is coupled to arectifying circuit 610, which in turn provides a dc output to aseries-coupled output inductor 612. Output inductor 612 is coupled to adc link 614 of the power conditioning unit, to which is also coupled adc link capacitor 616. A dc-to-ac converter 618 has a dc input from a dclink and provides an ac output 620, for example to an ac grid mainssupply.

A microcontroller 622 provides switching control signals to dc-to-acconverter 606, to rectifying circuit 610 (for synchronous rectifiers),and to dc-to-ac converter 618 in the output ‘unfolding’ stage. Asillustrated microcontroller 622 also senses the output voltage/currentto the grid, the input voltage/current from the PV module 602, and, inembodiments, the dc link voltage. (The skilled person will be aware ofmany ways in which such sensing may be performed). In some embodimentsthe microcontroller 622 implements a control strategy as previouslydescribed. As illustrated, the microcontroller 622 is coupled to an RFtransceiver 624 such as a ZigBee™ transceiver, which is provided with anantenna 626 for monitoring and control of the power conditioning unit600.

Referring now to FIG. 3 b, this shows details of a portion of an exampleimplementation of the arrangement of FIG. 3 a. This example arrangementemploys a modification of the circuit of FIG. 2a and like elements tothose of FIG. 2a are indicated by like reference numerals; likewise likeelements to those of FIG. 3a are indicated by like reference numerals.In the arrangement of FIG. 3b an LLC converter is employed (by contrastwith FIG. 2a ), using a pair of resonant capacitors C1, C3.

The circuits of FIGS. 1 to 3 are particularly useful for microinverters,for example having a maximum rate of power of less than 1000 Watts andor connected to a small number of PV modules, for example just one ortwo such modules. In such systems the panel voltages can be as low as 20volts and hence the conversion currents can be in excess of 30 amps RMS.

VAR Control Techniques

We will now describe embodiments of a modular adjustable power factorsolar inverter system which is able to track the power factor curve ofFIG. 1a and, more particularly, which is able to provide a power factorwhich is adjustable in the range plus/minus 0.95 cos φ with, inembodiments, an accuracy of 0.01 cos φ. A power conditioning unit (solarinverter) of the type described above, with a controllable currentsource (Q9) and a series output inductance in the above example)followed by a grid frequency unfolding rectifier is able to provide alagging power factor which can approach unity. This therefore addressesthe −0.95 cos φ requirement. Broadly speaking in embodiments of thesystem a leading power factor is provided by employing a switchedcapacitor arrangement (with control of the switching phase), but thereare some additional techniques which are employed to facilitate controland upgrade of a modular system.

Thus referring to FIG. 4 a, this shows an embodiment of a solar invertersystem 400 comprising a set of PV panels 402 each providing dc power toa respective solar inverter or power conditioning unit 404, preferredembodiments of the type described above. Each inverter provides an acgrid mains output to a shared ac connection 406 which provides a commongrid power feed 408. In embodiments of the system there are multiplesolar inverters, each with a relatively small power output, as describedfurther later. The system is controlled by system controller 410, forexample using a Zigbee™ wireless network coupling an antenna 412 of thesystem controller to each of the inverters.

Each inverter includes a switched capacitor coupled to a power factorcontroller under control of system controller 410. As previouslydescribed, each inverter controls the output current and reads the RMSoutput voltage and thus is able to determine the percentage of its fullpower that the inverter is providing and/or an absolute measure of thepower it is providing to the common grid tie 406, 408. Thus systemcontroller 410 is able to determine the absolute power provided to thegrid by each inverter and/or the percentage of an inverter's full powerbeing provided by each inverter. The system controller 410 uses thisinformation to control the switching of the capacitors in one or more ofthe inverters, as described later.

FIG. 4b illustrates a three phase solar inverter system 420 similar tothat of 4 a, in which like elements are indicated by like referencingnumerals. In the example of FIG. 4b one set of inverters 404 a, b, c,powered by respective sets of solar PV panels 402 a, b, c, drives eachphase of the grid mains, and each set of inverters driving each phase isseparately controlled by system controller 410.

In embodiments of the three phase system shown in FIG. 4b optionallyinverter modules 404 may provide information to the system controller410 which enables the system controller to determine which invertermodules are connected to which individual phase of the three phase gridsupply. This may comprise, for example, data identifying the timing ofthe of the sinusoidal voltage waveform on the ac output of the inverter,this information being sent to system controller 410 over the wirelessnetwork. From this timing information the system controller is able todetermine which inverters are connected to which phase and therefore isable to determine which sets of inverters are to be controlled togetherto control the displacement power factor for each phase. Alternatively,information defining which inverter is connected to which phase may beprogrammed into the system controller, for example on installation.

FIG. 4c illustrates an example installation of a system of the typeshown in FIGS. 4a and 4b (in which only one solar panel/inverter isshown for simplicity). In the arrangement of FIG. 4c the function of thesystem controller 410 is implemented in a gateway 430, here a wirelessbase station, which also provides an interface to the solar invertersystem. In this example the gateway 430 is coupled to a broadbandinternet modem/router 432 which provides a connection to the Internet434. This in turn provides a remote interface 436 for a monitoring site,for example for a user or system vendor or electricity supplier.

The arrangement of FIG. 4c shows the panels on the roof 442 of abuilding 440 with an electricity cable 406 to a consumer unit configuredto provide the grid interface 408. The illustrated example Zigbee™repeater 444 with one antenna 446 external to the building, and oneantenna 448 internal to the building, together with an optional signalsplitter 450 or other arrangement. This facilitates an RF signalpropagation between the system controller and the inverters, asdescribed in more detail in our co-pending U.S. patent application Ser.No. 13/244,222 (hereby incorporated by reference), which is useful wherethe roof incorporates conductive thermal insulation or, in a commercialbuilding, comprises corrugated metal sheets.

Referring now to FIG. 5 this shows a portion of an example solarinverter module 404 for the system of FIG. 4, in which like elements tothose previously described are indicated by like reference numerals.FIG. 5 shows just the output portion of the solar inverter previouslydescribed, that is the portion of the inverter from the dc link onwards.Because FIGS. 5a to 5c show a similar solar inverter modules with anincreasing level of detail, for simplicity these will be describedtogether.

Thus the inverter 404 comprises a dc link 614 with an energy storagecapacitor 616 which provides power to a current source stage 500, moreparticularly a voltage controlled current source providing power to abuck inductor assembly 612, and thence to a full wave rectifier outputunfolding stage 618. In embodiments, as previously described, thecurrent injection is regulated using current control in the inductorassembly 612 via an intermediate buck-type stage provided by currentsource 500 (this circuit block also includes a microcontroller andunfolding drivers, not explicitly illustrated for simplicity). Theinductor (output) current is sensed by resistor 504 and compared withthe reference to determine whether or not to provide current to inductor612, thus providing current mode control. Further details can be foundin our U.S. patent application Ser. No. 11/718,879, hereby incorporatedby reference.

The ac output is filtered by capacitor 506 and inductor 508 andprotected by varistor 510 and fuse 512 prior to ac grid mains output514. For simplicity details of the drivers for unfolding stage 618 areshown as part of circuit block 500, using an ac phase sense connection516 to synchronise with the grid.

Continuing to refer to FIG. 5, circuit block 500 also includes an output520 for controlling whether or not one or more capacitors 522 are to beswitched into parallel connection across ac output 514. The switchedcapacitor drive output 520 may, in embodiments, be provided as an outputfrom the microcontroller 622 illustrated in FIG. 3, receiving a signalfrom the system controller 410 via RF transceiver 624. The switchedcapacitor drive output 520 is provided to a switched capacitor controlcircuit block 530 via an isolation transformer 524, the control block530 responding to the switch signal to connect or disconnect capacitor522 across ac output 514. in embodiments the switched capacitor controlblock 530 may implement on/off control of switching of the capacitor522, or phase-sensitive control based on detection of 0-crossing of thecapacitor current (peak detection of the grid voltage).

FIG. 5c shows details of an embodiment of the arrangement using a triac540 to control switching of capacitor 522. Use of a triac is preferableas this is better able to handle a surge than a MOSFET. In this exampleimplementation a transistor stage 542 couples an output of transformer524 to a triac driver integrated circuit 544, for example an AVS08 fromSGS-Thomson or similar. In embodiments this is arranged to provide atrain of pulses to turn the triac on at the peak of the grid voltage (0capacitor current), although in principle because the triac will holditself on only one pulse need be employed if this is timed to turn thetriac on when the current through the triac is rising.

The circuit of FIG. 5c also able to provide variable phase control forthe switched capacitor drive output 520. This control enables theproportion of an ac cycle for which the capacitor 522 is connectedacross the ac mains to be varied, drive 520 thus determining aproportion of a cycle for which the capacitor is applied. This isachieved by controlling a switching point of the triac 540. in this waythe effective capacitance provided across the ac grid output of theinverter module may be varied, in embodiments linearly.

In one preferred method of triac control the triac is first switched inat the zero-crossing of the grid voltage. Then, for several grid cycles,for example of order 50 cycles, the triac is driven with a continuouspulse train to assure firing at all angles while the switching currenttransient reduces. After the switching transient has reduced tonegligible levels, the triac may be driven either with a single pulse ora pulse train only near the peak of the grid voltage.

As previously described, in preferred embodiments the inverter has theability to sense or otherwise determine the phase of the grid voltage.Although this is not essential it is helpful, in particular forcontrolling triac switching. Sensing the phase of the grid voltage canhelp to assure adequate firing of the triac during the startuptransient. This reduces stress on the VAR-control capacitor, byfacilitating switching in at the zero-crossing. It also reduces powerloss—by reducing the triac drive to the zero-crossing of the current,which corresponds to the peak and trough of the grid voltage after theinitial current transient has been reduced to negligible levels.

The example circuits of FIG. 5 merely illustrate one preferredembodiment of a solar inverter module for use in the system we describe.The skilled person will appreciate that alternative approaches may alsobe employed and that, in particular, embodiments of the technique wedescribe for power factor control are not restricted to use with a solarinverter employing a current source followed by an inductor assembly.

Referring next to FIG. 6, this shows an embodiment of a procedureoperating on system controller 410 which controls the solar invertermodules to switch the output capacitors in or out so that the system isable to provide a power factor which varies from unity to plus 0.95 cosφ leading. In broad terms, capacitance of 0, 1 or more modules areswitched into the grid as required based on the output export power ofthe system, as this varies from a minimum to a maximum. Because thesystem comprises multiple micro-inverters connected in parallel feedinga common grid tie point (per phase in the case of a three phase system)the required capacitance is also distributed amongst the modules and if,for example, the system is extended by adding more modules, the abilityto correct the power factor is also, automatically, increased.

Thus referring to FIG. 6, after initialisation (S650) the systemcontroller polls each inverter to read the net power output from theinverter via the Zigbee™ network (S652). The controller then sums thepower reported by each inverter module (S656) and calculates the powerbeing provided by the system as a percentage of the total output power.The power read from each inverter may either be an absolute power or apercentage of the total power that particular inverter is able toprovide; in this latter case the percentages may be averaged todetermine the total output percentage of the system. As previouslymentioned, the power factor of an inverter module (not including thecapacitor) is substantially unity, but not precisely unity. Thus theremay be an optional step (S654) to adjust for this non-unity power factorby means of a lookup table. This may either be performed at the inverteror in the system controller.

Once the total output power of the system as a percentage of the maximumexport power is determined, the system can then lookup or otherwisedetermine the required system power factor (S658), for example toapproximate the curve of FIG. 1 a. The system then determines the numberof inverters on which to switch in the output capacitance, to achievethis (S660). In one embodiment each inverter module has the same outputcapacitance, provided as a single switched output capacitance, but inalternative approaches an inverter may provide two or more switchablecapacitances and/or different inverter modules may have differentlysized switchable capacitances. The skilled person will appreciate thataccount may be taken of these variations in step 660. The systemcontroller then sends capacitor switch command signals to acorresponding number of inverters (S662), and the procedure loops backto step S652.

Where phase control of the output control capacitor is provided by aninverter module step S660 may also determine a proportion of the cyclefor which the capacitor is applied, this information being transmittedto the inverter modules at step S662.

FIG. 7a shows examples of the results of the procedure of FIG. 6 for asolar inverter system of the type illustrated in FIG. 4. Thestaircase-form curve 700 illustrates the effect of on/off capacitorcontrol and the linear curve 702 illustrates the effect of phase(proportional) control of the switched capacitors of the invertermodules. FIG. 7b illustrates the error in displacement power factor (cosφ) for the staircase curve of FIG. 7 a, showing the variation of thiserror with overall system power exported.

As can be seen from FIG. 7 and the preceding description, the powerfactor is controlled in steps (or linearly) by switching capacitorsinside each inverter at the appropriate system power level. Inembodiments of the system the net displacement power factor can becontrolled in theory to within an arbitrary tolerance limit by using thepower output of each inverter to be small enough to ensure an adequatenumber of steps. Thus for example for a system to provide a netdisplacement power factor within the 1% of a required value each stepshould cause the displacement power factor to change by at most 2% (thuschanging from the target value −1% to the target value +1%). Preferablythis step size should be a little smaller than this limit, toaccommodate tolerances and hysteresis in the system.

In the example given above, to satisfy VDE4105 with a 3.68 kVA systemthe (maximum) inverter size is 736 Watts, leading to a system with 5such inverters. In practice it is preferable to use inverters withslightly lower power and/or to provide each inverter with two or morecapacitor steps, to allow some margin for error. Thus in this example apractical upper limit in inverter size for each inverter isapproximately 480 Watts for inverters that each employ just a singleswitched VAR control capacitance.

Continuing the above example, for systems above 13.8 kVA in overalloutput power level, the VAR compensation required is twice that at 3.6kVA to 13.8 kVA (a compensation of cos φ up to 0.10 rather than 0.05).The inverters can be arranged to provide this level of VAR compensationeither by providing two separately switchable VAR control capacitancesin each inverter module and/or by using smaller inverters in the system,for example in the range 240 Watts-300 Watts.

Broadly speaking we have described a solar inverter system in which oneor more switched VAR control capacitances are provided in each solarinverter module, sized according to the maximum output power of theinverter module, to achieve a desired power factor compensation target.These are combined together in a system with a system controller whichis able to remotely control the addition of this capacitance and/or thephase of the capacitance, thus controlling the overall power factor ofthe system. Thus, for example, embodiments of this technique are able tocontrol the displacement power factor of the overall system with anaccuracy of 0.01 cos φ, or better. Embodiments of the system are closedloop in the sense that they monitor inverter power output into thecommon grid connection and provide control data back to the invertermodules for controlling the addition/phase of the switched capacitance,but are open loop in the sense that, in embodiments, no measurement isneeded of the power factor at the grid connection. The techniques wehave described are applicable to both single phase and three phasearchitectures.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

1-20. (canceled)
 21. A distributed energy generation inverter system forapplying volt-ampere reactive (VAR) control to an alternating current(AC) output, the distributed energy generation inverter systemcomprising: a plurality of individual inverters, each individualinverter comprising a direct current (DC) power input for receiving DCpower from a DC source, an AC power output for supplying AC power to acommon AC circuit, and a reactive element switchable to be connected inparallel to the AC output; and a controller communicatively coupled toeach of the plurality of inverters, the controller configured to monitorthe individual power output of each inverter, determine a fraction ofthe maximum total potential power being supplied by the plurality ofinverters, and when the fraction is above a pre-determined threshold,selectively controlling one or more of the individual inverters toswitch their respective reactive elements into parallel connectionacross the respective AC outputs.
 22. The distributed energy generationinverter system of claim 21, wherein the controller employs phasecontrol to selectively control the one or more of the individualinverters to switch the respective reactive elements in parallel for aportion of an AC cycle in which the reactive element is switched in. 23.The distributed energy generation inverter system of claim 21, whereinthe common AC circuit is a three phase AC power feed, and wherein aninverter of the plurality of inverters is for coupling to each phase ofthe three phase AC power feed, the controller configured to control apower factor of each phase of the three phase AC power feed.
 24. Thedistributed energy generation inverter system of claim 21, wherein thereactive element includes a capacitor.
 25. The distributed energygeneration inverter system of claim 21, wherein the controller iscommunicatively coupled to each of the plurality of inverters by way ofa wireless communication channel.
 26. The distributed energy generationinverter system of claim 21, wherein the DC power source is a solarphotovoltaic panel.
 27. The distributed energy generation invertersystem of claim 21, wherein the pre-determined threshold is greater thanor equal to 0.5.
 28. The distributed energy generation inverter systemof claim 21, wherein the controller is configured to access a lookuptable that stores power factor compensation data defining a number ofreactive elements to switch in parallel to the fraction of the maximumtotal potential power being supplied by the plurality of inverters. 29.The distributed energy generation inverter system of claim 21, whereinmonitoring the individual power output of each inverter includes sensinga current provided by each of the inverters to the common AC circuit.30. The distributed energy generation inverter system of claim 21,wherein each of the individual inverters includes a switching devicecoupled to switch their respective reactive element, and wherein thecontroller is configured to switch the switching device at a peakvoltage point of the AC power.
 31. The distributed energy generationinverter system of claim 29, wherein each of the switching devicesincludes a triac.
 32. The distributed energy generation inverter systemof claim 21, wherein the controller includes a field-programmable gatearray (FPGA).
 33. The distributed energy generation inverter system ofclaim 21, wherein each of the individual inverters includes a two-stagepower converter, wherein a first stage of the two stage power converteris electrically isolated from the second stage of the two stage powerconverter.
 34. The distributed energy generation inverter system ofclaim 33, wherein the first stage is to be coupled to the DC powersource, and wherein the first stage include a switching DC-to-ACconverter.
 35. The distributed energy generation inverter system ofclaim 21, wherein each of the plurality of individual inverters includesa second reactive element switchable to be connected in parallel to theAC output, and when the fraction is above a pre-determined threshold,the controller is further configured to selectively control one or moreof the individual inverters to switch their respective second reactiveelements into parallel connection across the respective AC outputs. 36.An controller for a distributed energy generation system for applyingvolt-ampere reactive (VAR) control to an alternating current (AC)output, the controller comprising: a communication interface forcommunicating with a plurality of inverters; processing logic coupled tothe communication interface; and a computer-readable medium accessibleto the processing logic, the computer-readable medium storinginstructions, when executed by the processing logic will cause thecontroller to perform a method comprising: monitoring, via thecommunication interface, the individual power output of each of theplurality of inverters; determining a fraction of the maximum totalpotential power being supplied to a common AC power grid by theplurality of inverters; selectively switching, via the communicationinterface, reactive elements in one or more of the plurality ofinverters when the fraction is above a pre-determined threshold, whereinselectively switching the reactive elements puts the reactive elementsin parallel connection with the AC power grid to apply volt-amperereactive (VAR) control to an AC output to the AC power grid.
 37. Thecontroller of claim 36, wherein selectively switching the reactiveelements includes switching the reactive elements for a portion of an ACcycle to facilitate phase control.
 38. The controller of claim 36,wherein the communication interface is a wireless communicationinterface.
 39. The controller of claim 36, wherein the pre-determinedthreshold is greater than or equal to 0.5.
 40. The controller of claim36, wherein the computer-readable medium includes a lookup table thatstores power factor compensation data defining a number of reactiveelements to switch in parallel to the fraction of the maximum totalpotential power being supplied by the plurality of inverters.